High data rate wireless bridging

ABSTRACT

A specialized preamble is provided to facilitate matrix channel estimation of a MIMO channel. In a particular implementation, a channel training preamble provided by the IEEE 802.11a standard is modified to facilitate MIMO channel estimation.

STATEMENT OF RELATED APPLICATIONS

This patent application is a divisional of U.S. patent application Ser.No. 13/664,218, filed Oct. 30, 2012, which is a continuation of U.S.patent application Ser. No. 12/011,546, filed Jan. 28, 2008 (U.S. Pat.No. 8,374,105), which is a divisional of U.S. patent application Ser.No. 10/335,500, filed Dec. 31, 2002 (U.S. Pat. No. 7,352,688), which areincorporated herein by reference in their entirety.

The present application is related to the subject matter of:

U.S. Pat. No. 6,377,631, issued on Apr. 23, 2002, entitled “TRANSMITTERINCORPORATING SPATIO-TEMPORAL PROCESSING;”

U.S. patent application Ser. No. 10/197,300, filed Jul. 15, 2002,entitled “MEDIA ACCESS CONTROL FOR MIMO WIRELESS NETWORK” (U.S. Pat. No.7,301,924);”

U.S. patent application Ser. No. 10/207,694, filed Jul. 29, 2002,entitled “POINT-TO-POINT MAC PROTOCOL FOR HIGH SPEED WIRELESS BRIDGING”(U.S. Pat. No. 7,567,537).

The contents of the above are incorporated herein by reference for allpurposes in their entirety.

BACKGROUND

The present invention relates generally to communications and moreparticularly to systems and methods for wireless communications.

As the Internet continues its development and as workers and consumersincreasingly rely on data networking to assist their day-to-day tasks, aneed arises to extend network connectivity to locations where there isno convenient connection to a wired infrastructure. Workers desire tosend and receive email and access the Internet and their corporateintranet even when they are away from their workstation. Consumers wishto establish home networks without costly and cumbersome wiring.Accordingly, wireless communication standards have evolved including theIEEE 802.11 family.

The current IEEE 802.11a standard allows for wireless communicates atspeeds between 6 Mbps and 54 Mbps. It is desirable to further increasethese speeds to accommodate delivery of multimedia wireless services andfacilitate outdoor wireless bridging between indoor networks. It is alsodesirable to accommodate the increased data rates by increasing spectralefficiency rather than by increasing bandwidth.

One known way of increasing spectral efficiency is the use of MIMO(Multiple Input Multiple Output) processing techniques. MIMO techniquestake advantage of multiple antennas (or multiple polarizations of thesame antenna) at the transmitter and receiver to access multiple channelinputs and channel outputs and thereby define multiple spatialsubchannels that occupy the same bandwidth but nonetheless are capableof carrying independent data streams. The delineation of the multiplespatial subchannels may involve weighting of the antenna inputs at thetransmitter end and/or weighting of the antenna outputs at the receiverend. For further information on MIMO techniques, see U.S. Pat. No.6,377,631.

It is desirable to apply MIMO techniques to IEEE 802.11 systems toincrease data carrying capacity, but there are obstacles to overcome.The 802.11 standards do not currently specify MIMO transmissiontechniques. In particular, MIMO transmission techniques preferably takeadvantage of an estimate of the MIMO channel response. The MIMO channelresponse is represented by a matrix composed of elements correspondingto each combination of channel input and channel output. By contrast,802.11a, for example, provides only for estimating a conventionalchannel response that assumes a single input and a single output.

It would be desirable to maximize the usage of 802.11 techniques andcomponents in a MIMO wireless communication system while still meetingthe MIMO requirement of obtaining a MIMO matrix channel estimate. It isfurthermore generally desirable to use MIMO techniques to increase thecapacity of 802.11 networks.

SUMMARY

By virtue of one embodiment of the present invention, a specializedpreamble is provided to facilitate matrix channel estimation of a MIMOchannel. In a particular implementation, a channel training preambleprovided by the IEEE 802.11a standard is modified to facilitate MIMOchannel estimation.

A first aspect of the present invention provides a method fortransmitting a signal. The method includes: transmitting a first OFDMsignal via a first antenna element, transmitting a second OFDMsignal—co-channel to the first OFDM signal—via a second antenna element,transmitting channel training information in the first OFDM signal whileinhibiting transmission of the second OFDM signal, and transmittingchannel training information in the second OFDM signal while inhibitingtransmission of the first OFDM signal.

A second aspect of the present invention provides a method for receivinga signal. The method includes: receiving a first OFDM signal via a firstantenna element, receiving a second OFDM signal via a second antennaelement, and during a first channel training period—within each of aplurality of frequency subchannels—obtaining received signalmeasurements via the first antenna element and via the second antennaelement, during a second channel training period—within each of aplurality of frequency subchannels—obtaining received signalmeasurements via the first antenna element and via the second antennaelement, and measuring a MIMO channel response based on the measurementsmade during the first channel training period and the second channeltraining period.

A third aspect of the present invention provides a method fortransmitting a signal. The method includes: transmitting a first OFDMsignal via a first antenna element, transmitting a second OFDM signal,co-channel to the first OFDM signal, via a second antenna element, andduring a channel training period, transmitting channel traininginformation simultaneously in a first set of subcarriers of the firstOFDM signal and in a second set of subcarriers of the second OFDMsignal, the first set of subcarriers and the second set of subcarriersbeing non-overlapping.

A fourth aspect of the present invention provides a method for receivinga signal. The method includes: receiving a first OFDM signal via a firstantenna element, receiving a second OFDM signal—co-channel to the firstOFDM signal—via a second antenna element, during a channel trainingperiod, obtaining received signal measurements in a first set ofsubcarriers and a second set of subcarriers, the first set ofsubcarriers and the second set of subcarriers being non-overlapping, andmeasuring a MIMO channel response based on the measurements made duringthe channel training period. Transmissions from different remote antennaelements are expected in the first set of subcarriers and the second setof subcarriers during the channel training period.

Further understanding of the nature and advantages of the inventionsherein may be realized by reference to the remaining portions of thespecification and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a wireless bridge configuration according to oneembodiment of the present invention.

FIG. 2 depicts a MIMO receiver according to one embodiment of thepresent invention.

FIG. 3 depicts a MIMO transmitter according to one embodiment of thepresent invention.

FIG. 4 is a flow chart describing steps of operating a MIMO receiveraccording to one embodiment of the present invention.

FIG. 5 depicts a preamble structure to facilitate MIMO channelestimation according to one embodiment of the present invention.

FIG. 6 depicts an alternative preamble structure to facilitate MIMOchannel estimation according to one embodiment of the present invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Although having very broad applicability, the present invention will bedescribed with reference to a representative network environment, awireless communication network based on the IEEE 802.11 standard, and inone particular implementation, the IEEE 802.11a standard. The IEEE802.11g standard is considered to be highly similar to the 802.11astandard in many respects and wherever the present application refers tothe 802.11a standard, it will be understood that this encompasses the802.11g standard as well. Familiarity with the technical details ofthese standards will be assumed in the discussion that follows. Relevantdescriptive materials regarding the IEEE 802.11 standards may be foundin the following documents:

Information technology—Telecommunications and information exchangebetween systems—Local and metropolitan area networks specificrequirements—Part 11: Wireless LAN Medium Access Control (MAC) andPhysical Layer (PHY) Specifications (1999).

Information technology—Telecommunications and information exchangebetween systems—Local and metropolitan area networks specificrequirements—Part 11: Wireless LAN Medium Access Control (MAC) andPhysical Layer (PHY) Specifications (1999): High Speed Physical Layer inthe 5 GHz Band, hereinafter “High Speed Physical Layer in the 5 GHzBand”.

The contents of these standards documents are herein incorporated byreference for all purposes in their entirety.

The IEEE 802.11a standard employs orthogonal frequency divisionmultiplexing (OFDM) as known in the art. In OFDM, the availablebandwidth is effectively divided into a plurality of subchannels thatare orthogonal in the frequency domain, each such subchannel beingoccupied by a “subcarrier” or tone. For each successive OFDM symbol, acomplex value is assigned to each subcarrier. To create the basebandtime domain signal for transmission, an IFFT is applied to a series of64 complex subcarrier values to obtain 64 time domain samples. In802.11a, some of the values are always zero and others carry pilot tonesused for phase synchronization. The resulting series of time domainsamples is augmented with a cyclic prefix prior to transmission. The useof the cyclic prefix assures the orthogonality of the subcarriers. Thecyclic prefix addition process can be characterized by the expression:[z(1) . . . z(N)]^(T) →[z(N−v+1) . . . z(N)z(1) . . . z(N)]^(T)

FIG. 1 depicts a representative wireless communication link 100 to whichembodiments of the present invention may be applied. Communication linkinterconnects two nodes 102 and 104 which may be wireless bridges. Inone embodiment, wireless bridges 102 and 104 interoperate in accordancewith the IEEE 802.11 standard, enhanced and extended as describedherein. In one particular implementation, the physical layer interactionis defined by the IEEE 802.11a standard. Any wireless communicationprotocol suite consistent with the present invention may be used.

Each of the bridges is equipped with two antenna elements 106H and 106V.These two antenna elements may represent the horizontal and verticalpolarizations of the same antenna or may represent two differentantennas. By use of two antenna elements on each bridge, a multipleinput multiple output (MIMO) channel is established between the bridges.Any number of antenna elements may be used within the scope of thepresent invention. Here the MIMO channel has two inputs and two outputsbut the number of either inputs or outputs may be increased further inaccordance with the present invention.

To enhance information carrying capacity, distinct signals aretransmitted via the two antenna elements on one bridge. The other bridgeis able to receive two distinct signals by appropriate weighting of thesignal received on its own two antenna elements. The two transmittedsignals share the same frequency allocation yet nonetheless may berecovered separately. In this way, two independent spatial subchannelsare established and the data carrying capacity of the wirelesscommunication link may be doubled. For example, in an IEEE 802.11asystem, the maximum data carrying capacity may be increased from 54 Mbpsto 108 Mbps. This is particularly useful in connecting wireless bridgesas are used for backhaul and outdoor interbuilding links.

FIG. 2 depicts a MIMO receiver 200 as would be included in one of thebridges of FIG. 1. The horizontal and vertical antenna polarizations106H and 106V are coupled to separate demodulators/analog to digitalconverters 202 respectively. These blocks perform RF amplification andfiltering, downconversion to an intermediate frequency (IF), IFfiltering, downconversion to baseband, and conversion to digital form.Offset estimation blocks 203 measure the phase and frequency offsets ofthe received signal relative to local timing based on preamble structureas will be explained below. One receiver chain acts a timing master sothat its measured offset serves to synchronize both chains. A controller207 manages the process of selecting the timing master and passingsynchronization to the other receiver chain which acts as a slave.Blocks 205 shift the frequency and phase of the received signal based onthe measured offsets for synchronization to local timing. Global timingbased on the guard band is also handled in this way, being determined bythe master and transferred to the slave.

Since the IEEE 802.11a standard employs orthogonal frequency divisionmultiplexing (OFDM), the next step is to convert the time domain digitaldata streams into the frequency domain using FFT blocks 204. The cyclicprefixes associated with each OFDM symbol are removed prior toprocessing by FFT blocks 204. The output of each FFT block 204 is astream of complex signal values received in each of the 64 subcarriers.

As will be explained below, a channel estimation block 206 determinesthe MIMO matrix channel response based on measurements made duringspecial channel training periods when channel training information issent. The matrix channel estimate is provided on asubcarrier-by-subcarrier basis. A beamforming block 208 appliesweightings determined based on the channel estimates to the receivedhorizontal and vertical data streams to obtain the two paralleltransmitted data streams. These data streams are then subject to furtherdecoding, deinterleaving, and descrambling by decoders 210.

FIG. 3 depicts a MIMO transmitter system 300 according to one embodimentof the present invention. In this depicted implementation, signals forthe horizontal and vertical antenna polarizations are generatedseparately in parallel. It is also possible to employ spatial processingat the transmitter end by defining antenna element weights correspondingto two spatial subchannels. Here, instead a first spatial subchannel issimply mapped to the horizontal antenna polarization and a secondspatial subchannel is mapped to the vertical antenna polarization.

Data is first scrambled, encoded, interleaved in an encoding block 302as defined by the IEEE 802.11a standard. Modulators 304 translate thecoded data bits into complex values to be assigned to OFDM subcarriersin accordance with the currently selected modulation scheme (4-QAM,16-QAM, etc.) A pilot tone insertion block 306 inserts pilot tones atsubcarrier positions defined by the IEEE 802.11a standard to supportphase offset synchronization at the receiver. IFFT blocks 308 convertgroups of 64 subcarrier values from a frequency domain to the timedomain. The output is a succession of OFDM symbols carrying payloaddata.

The IEEE 802.11a standard also provides for the use of a preambleincluding special symbols for use in synchronization and carrierestimation. In a transmitted frame or packet, this preamble precedes thedata-carrying OFDM symbols. The preamble includes so-called short andlong symbols. The portion of the packet including the long symbols ismodified to facilitate MIMO channel estimation as it will be explainedin greater detail below. Insertion blocks 310 insert the preamblesymbols.

The digital signal generated by insertion block 310 is converted toanalog by digital to analog converters 312. The complex basebandwaveforms output by converters 312 are upconverted to an intermediatefrequency (IF), amplified and filtered at the IF, converted to thetransmission radio frequency (RF), further amplified and filtered, andthen transmitted via the appropriate antenna element. A block 314represents the various analog processing steps.

FIG. 4 is a flowchart describing steps of MIMO receiver operation withreference to a single received packet according to one embodiment of thepresent invention. Two parallel flows are presented corresponding to thetwo parallel receiver chains. At step 402A one of the receiver chains isthe first to detect start of packet (SOP). The receiver chain that isthe first to detect start of packet becomes the “master” for timing andfrequency synchronization purposes while the receiver chain thatsubsequently detects start of packet at step 402B is the timing “slave”.Then at steps 404A and 404B, both receiver chains separately perform anautomatic gain control operation, setting amplifier gain in accordancewith received signal strength. This operation takes place within blocks202.

At step 406A, the receiver chain that has been designated as the masterestimates the frequency offset based on the short symbols in thepreamble using block 203 by employing the short symbol structurespecified by the 802.11a standard and corrects for this offset usingblock 205. Alternatively, a local oscillator within block 202 may beadjusted to effectively shift the received frequency. The frequencyoffset that has been determined is transmitted to the slave receiverchain via controller 207. At step 406B, the slave receiver chaincorrects its frequency offset using block 205. At step 408A the masterreceiver chain estimates global timing based on detection of a guardinterval within the preamble and corrects its timing accordingly. Thetiming reference is sent to the slave receiver chain and the slavereceiver chains synchronize its own timing at step 408B.

At step 410A and 410B, the receiver chains make measurements during afirst long symbol field of the preamble. At steps 412A and 412B, thereceiver chains make measurements during a second long symbol field ofthe preamble. At steps 414A and 414B, the channel estimation block 206estimates the MIMO matrix channel based on the measurements of the twopreceding steps. Steps 414A and 414B also obtains weightings among theantenna elements that will define independent received spatialsubchannels. A so-called “SIGNAL” field follows the preamble. This fieldcontains information about the modulation type to be used and theoverall length of a packet. The “SIGNAL” field is processed at steps416A and 416B. This is followed by the processing of data-carrying OFDMsymbols at steps 418A and 418B.

A more detailed description of the MIMO channel estimation procedurewill benefit from an understanding of the modified preamble contents.FIG. 5 depicts a MIMO preamble structure according to one embodiment ofthe present invention. The transmitter antenna elements do not sendidentical preambles but rather the preamble structure is differentiatedbetween them to facilitate estimation of the MIMO channel. In FIG. 5,the top preamble is for a transmitter antenna element referred to asantenna element 1 while the bottom preamble refers to an antenna element2. The two preambles are transmitted simultaneously.

Each preamble begins with 10 short symbols 502 that are specified by the802.11a standard and that are used for signal detection, automatic gaincontrol, and frequency offset estimation. Each short symbol is specifiedto be 0.8 microseconds long.

A channel training period follows the short symbols. During a firstportion of the channel training period, one antenna element transmits aguard interval 504 followed by two long symbols 506 while the otherantenna element is quiet during this period. Then the first antennaelement goes into a quiet period while the second antenna element,following a guard interval 508, also transmits two long symbols 510.

Both guard intervals are specified to be 1.6 microseconds long whileeach long symbol is specified to be 3.2 microseconds long. The longsymbol values are preferably the same as those specified by the 802.11astandard. (See page 13 of “High Speed Physical Layer in the 5 GHzBand”). Here, however, the preamble is extended to allow independentmeasurement of the characteristics of each channel input output pair.After both antenna elements have transmitted their long symbols and thuscompleted the channel training period, they transmit signal fields 512that designate modulation type and packet length followed by independentdata streams 514.

Although the short symbol and long symbol are specified by the IEEE802.11a standard in terms of their subcarrier values, it is alsopossible to precompute and store time domain sample values. This is whysymbol insertion block 310 is shown after the IFFT stage in FIG. 3. Itis also possible to use the specified frequency domain subcarrier valuesand insert these prior to IFFT block 308.

Further explanation of MIMO channel estimation and MIMO receiverprocessing will benefit from a mathematical model of the MIMO channel.All values are assumed to be complex and to vary over subcarrierposition. A linear system model is given as follows:

$\begin{bmatrix}{Y_{1}(k)} \\{Y_{2}(k)}\end{bmatrix} = {{\begin{bmatrix}{H_{1,1}(k)} & {H_{1,2}(k)} \\{H_{2,1}(k)} & {H_{2,2,}(k)}\end{bmatrix}\begin{bmatrix}{Z_{1}(k)} \\{Z_{2}(k)}\end{bmatrix}} + \begin{bmatrix}{N_{1}(k)} \\{N_{2}(k)}\end{bmatrix}}$ Y(k) = H(k)Z(k) + N(k)where Y₁(k) and Y₂(k) are received signals on the two receiver antennaelements (1 and 2) for a tone or subcarrier k and Z₁(k) and Z₂(k) aretransmitted signals on the two transmitter antenna elements (1 and 2).N₁(k) and N₂(k) refer to the noise received on the two receiver antennaelements.

The goal of channel estimation is to obtain the contents of the H matrixfor each tone k. This is calculated based on received tone values duringthe long training symbols 506 and 510. Based on the first two longtraining symbols (transmitted by transmitter antenna element 1), thefollowing may be obtained:

${{\hat{H}}_{1,1}(k)} = \frac{{Y_{1}\left( {k,1} \right)} + {Y_{1}\left( {k,2} \right)}}{2 \cdot {{LS}(k)}}$${{\hat{H}}_{2,1}(k)} = \frac{{Y_{2}\left( {k,1} \right)} + {Y_{2}\left( {k,2} \right)}}{2 \cdot {{LS}(k)}}$

where Y₁(k,1) and Y_(i)(k,2) refer to the complex subcarrier valuesreceived via receiver antenna element 1 for tone k during the two longtraining symbols 506 respectively, and Y₂(k,1) and Y₂(k,2) refer to thecomparable values for receiver antenna element 2. The term LS (k) refersto the known transmitted long training symbol value at tone k.

The two remaining H matrix values then are obtained from the longtraining symbols 510 transmitted by the second transmitter antennaelement. These values are given as:

${{\hat{H}}_{1,2}(k)} = \frac{{Y_{1}\left( {k,1} \right)} + {Y_{1}\left( {k,2} \right)}}{2 \cdot {{LS}(k)}}$${{\hat{H}}_{2,2}(k)} = \frac{{Y_{2}\left( {k,1} \right)} + {Y_{2}\left( {k,2} \right)}}{2 \cdot {{LS}(k)}}$

To suppress noise in the channel estimate, it is desirable to apply tonesmoothing to the channel estimate. This may be done by using, e.g., athree-tap filter. The smoothed channel estimate can be given by:

$\begin{matrix}{{{{\overset{\sim}{H}}_{i,j}(k)}} = \frac{\sum\limits_{m = {- 1}}^{1}{a_{m} \cdot {{{\hat{H}}_{i,j}\left( {k - m} \right)}}}}{\sum\limits_{m = {- 1}}^{1}a_{m}}} & {{{- 25} \leq k \leq {- 2}},{2 \leq k \leq 25}} \\{{{angle}\mspace{14mu}\left\{ {{\overset{\sim}{H}}_{i,j}(k)} \right\}} = \frac{\begin{matrix}{\sum\limits_{m = {- 1}}^{1}{a_{m} \cdot {unw\_ ang}}} \\\left\{ {{\hat{H}}_{i,j}\left( {k - m} \right)} \right\}\end{matrix}}{\sum\limits_{m = {- 1}}^{1}a_{m}}} & {{{- 25} \leq k \leq {- 2}},{2 \leq k \leq 25}} \\{{{\overset{\sim}{H}}_{i,j}(k)} = {{\hat{H}}_{i,j}(k)}} & {{k = {- 26}},{- 1},1,26}\end{matrix}$

Note that in the calculation of angle{{tilde over (H)}_(i,j)(k)}, theangles of Ĥ(k) should be unwrapped prior to the calculation. This isdenoted by the function “unw_ang”. This means that as phases rotateacross the tone measurements, their values are set to increase ordecrease without bound rather than rotating through a 2π range. The tonesmoothing weights are denoted by a_(m). In an outdoor application wherethe frequency response of the channel will be slowly varying from toneto tone, one can set a_(m)=1 for maximal gain with a three-tap filter.With more frequency selectivity, one can set a⁻¹=1, a₀=2, and a₊₁=1 forsomewhat less smoothing. In order to not delay the decoding of theSIGNAL symbol, a non-smoothed channel estimate average can be usedduring the SIGNAL interval.

Having these smoothed channel estimates, weights can be generated basedon a variety of approaches. In one implementation, a zero-forcingapproach is used and weights are obtained as follows:

$\begin{bmatrix}{W_{1,1}(k)} & {W_{1,2}(k)} \\{W_{2,1}(k)} & {W_{2,2,}(k)}\end{bmatrix} = {\left\lbrack \begin{matrix}{{\overset{\sim}{H}}_{2,2}(k)} & {- {{\overset{\sim}{H}}_{1,2}(k)}} \\{- {{\overset{\sim}{H}}_{2,1}(k)}} & {{\overset{\sim}{H}}_{1,1}(k)}\end{matrix} \right\rbrack*\frac{1}{{{{\overset{\sim}{H}}_{1,1}(k)}{{\overset{\sim}{H}}_{2,2}(k)}} - {{{\overset{\sim}{H}}_{1,2}(k)}{{\overset{\sim}{H}}_{2,1}(k)}}}}$$\mspace{79mu}{{W(k)} = {{\overset{\sim}{H}}^{- 1}(k)}}$

Received signals can then be processed by beamformer 208 by use of:

$\begin{bmatrix}{{\hat{Z}}_{1}(k)} \\{{\hat{Z}}_{2}(k)}\end{bmatrix} = {\begin{bmatrix}{W_{1,1}(k)} & {W_{1,2}(k)} \\{W_{2,1}(k)} & {W_{2,2,}(k)}\end{bmatrix}\begin{bmatrix}{Y_{1}(k)} \\{Y_{2}(k)}\end{bmatrix}}$ Ẑ(k) = W(k)Y(k)where {circumflex over (Z)}₁(k) and {circumflex over (Z)}₂(k) are theestimates of the data transmitted by transmitter antenna elements 1 and2 on tone k. Alternative weight generating schemes can also be used suchas a minimum mean square error approach (see U.S. Pat. No. 6,377,631).

In an alternative variant, the preamble structure of FIG. 5 is modifiedso that four or more long symbols are used on each transmitting antennaelement. Channel estimation can thus be based on a longer measurementtime span. This provides an increase in accuracy of the channel estimateat the expense of greater preamble overhead. Overall link overhead maybe reduced, however, by use of the techniques presented in “MEDIA ACCESSCONTROL FOR MIMO WIRELESS NETWORK” as identified above.

FIG. 6 depicts another alternative preamble structure. Like thestructure of FIG. 5, each of the two preambles begins with the shortsymbols 502. Here, however, the two transmitter antenna elementstransmit long symbols simultaneously but using nonoverlapping subsets ofsubcarriers. For example, following the short symbols and a guardinterval 602, the first transmitter antenna element sends two longsymbols 604 where the even subcarriers are set to their specified valueswhile the odd subcarriers are set to zero. Simultaneously, the secondtransmitter antenna element transmits two successive long symbols 606where the odd subcarriers are set to their specified values while theeven subcarriers are set to zero.

Receiver processing is similar to what was described in relation to FIG.5 for channel estimation. Here, the received long symbol complexsubcarrier values are interpolated so that a received value is assignedto each subcarrier during each received long symbol even though onlyalternating subcarriers were transmitted. Any suitable interpolationscheme may be used in accordance with the present invention. Afterinterpolation, channel estimation processing proceeds as above. Thistechnique shortens the preamble to the length specified by the 802.11astandard without decreasing the effective time interval used for MIMOchannel estimation. Further increase in channel estimation accuracy maybe obtained by repeating the simultaneously transmitted long symbolswith non-overlapping sets of active subcarriers one or more times asdesired.

In the embodiments that have just been presented, both MIMO spatialsubchannels carry the same data rate. However, it is possible to varythe amount of data carried by each subchannel in accordance with thesignal to noise ratio or other index of channel quality as determinedfor each spatial subchannel. The table below presents data rates for asingle spatial subchannel and the required signal to interference plusnoise ratio (SINR) assuming a maximum 1% packet error rate, 3000 byteMAC layer packets split into two 1500 byte packets for transmission viaeach spatial subchannel, and a minimum of 15 dB of isolation between thespatial subchannels.

Data Rate Mbps per spatial subchannel Required SINR (dB) 6 8.5 9 9.0 1210.25 18 12.0 24 14.5 36 17.75 48 21.5 54 23.0

SINR information based on averages of the H_(1,1) and H_(2,2) channelestimates may be used to determine appropriate data rates for eachspatial subchannel. These data rates may be determined at the receiverend and sent back to the transmitter in a special MAC layer messageaccording to one embodiment of the present invention. Alternatively,SINR values may be sent back to the transmitter and the appropriate datarates for each spatial subchannel may be determined there.

If isolation between the spatial subchannels slips below 10 dB, it ispreferable to abandon MIMO operation and instead send a single datastream using, e.g., a single transmitter element and one receiverelement. It is also possible to use two receiver elements for diversityreception in SISO mode. Criteria for switching between MIMO and singleinput single output (SISO) operation are discussed in “POINT-TO-POINTMAC PROTOCOL FOR HIGH SPEED WIRELESS BRIDGING.”

In one implementation, a receiver may detect SISO operation byattempting SISO processing in parallel with MIMO receiver processing. Ifdata is successfully recovered using SISO operation when additional MIMOpreamble information is expected, the SISO data recovery processingcontinues and MIMO processing is terminated. Returning again to FIG. 4,SISO receiver processing, e.g., processing through a single antennaelement and receiver chain begins at step 420 in parallel withmeasurements made on the second long symbol field that would be expectedin a MIMO signal. If step 420 successfully recovers data, as determinedby decoder check sums, SISO receiver operation continues at step 422 andfurther MIMO processing is terminated.

It is understood that the examples and embodiments that are describedherein are for illustrative purposes only and that various modificationsand changes in light thereof will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication and scope of the appended claims and their full scope ofequivalents.

The invention claimed is:
 1. In a communication link capable of bothMIMO and SISO operation, apparatus for operating a receiver, saidapparatus comprising: a first receiver block that receives a first OFDMsignal via a first antenna element; a second receiver block thatreceives a second OFDM signal via a second antenna element; and achannel estimation block that during a first channel training period,within each of a plurality of frequency subchannels, obtains receivedsignal measurements via said first antenna element and via said secondantenna element; and wherein after said first channel training periodand during a second channel training period, said first OFDM signal isprocessed to recover data; and wherein only if data is not recoveredduring said second channel training period, said first OFDM signal andsaid second OFDM signal are thereafter processed as a MIMO signal. 2.The apparatus of claim 1 wherein channel training informationtransmitted during said first channel training period and said secondchannel training period is defined by an IEEE 802.11 standard.
 3. Theapparatus of claim 1 wherein said first antenna element represents ahorizontal polarization of an antenna and said second antenna elementrepresents a vertical polarization of said antenna.
 4. The apparatus ofclaim 1 wherein a first transmitting antenna element is quiet during aportion of said first and second channel training periods and a secondtransmitting antenna element is quiet during another portion of saidfirst and second channel training periods.
 5. The apparatus of claim 1wherein components of said first receiver block and said second receiverblock are synchronized.
 6. The apparatus of claim 5 wherein said firstreceiver block operates as a synchronization master and said secondreceiver block operates as a synchronization slave based on a start of apacket being detected first by said first receiver block.
 7. In a MIMOcommunication link, a method of operating a receiver, said methodcomprising: using a first receiver chain and a second receiver chain toprovide synchronization information, said first receiver chain coupledto a first antenna element, said second receiver chain coupled to asecond antenna element; and synchronizing components of said first andsecond receiver chains based on said synchronization information;wherein said first antenna element represents a horizontal polarizationof an antenna and said second antenna element represents a verticalpolarization of said antenna.
 8. The method of claim 7 wherein saidsynchronization information comprises an estimate of synchronizationparameters.
 9. The method of claim 7 wherein said first receiver chainoperates as a synchronization master and said second receiver chainoperates as a synchronization slave based on a start of a packet beingdetected first by said first receiver chain.
 10. A method comprising:receiving a first signal via a first antenna element at a MIMO (MultipleInput Multiple Output) receiver; receiving a second signal via a secondantenna element at the MIMO receiver; processing said signals at firstand second receiver chains; transmitting frequency offset informationfrom at least one of said receiver chains; and estimating a frequencyoffset.
 11. The method of claim 10 wherein one of said first and secondreceiver chains operates as a master and the other of said first andsecond receiver chains operates as a slave.
 12. The method of claim 11wherein the master receiver chain is the first of said receiver chainsto detect a start of a packet.
 13. The method of claim 10 wherein saidfirst antenna element represents a horizontal polarization of an antennaand said second antenna element represents a vertical polarization ofsaid antenna.
 14. The method of claim 10 further comprisingsynchronizing components of said first and second receiver chains. 15.The method of claim 14 wherein said synchronization is based onsynchronization information transmitted between said receiver chains,said synchronization information comprising an estimate ofsynchronization parameters.
 16. The method of claim 10 furthercomprising correcting frequency offset calculated at one of saidreceiver chains based on said frequency offset information received fromthe other of said receiver chains.
 17. The method of claim 10 furthercomprising measuring a MIMO channel response.
 18. The method of claim 10further comprising estimating a MIMO channel matrix.
 19. The method ofclaim 10 wherein said frequency offset is applied at each of saidreceiver chains.